Cooling technique

ABSTRACT

A method and an apparatus are provided for cooling a heat source using a refrigerant flow in a heat exchanger. According to some embodiments, a flow property of the refrigerant as it flows through a heat exchanger is measured and based on the measurement a flow distribution rate for the flow of the refrigerant in the heat exchanger is determined. A valve is operated for adjusting the flow rate of the refrigerant in the heat exchanger according to the determined flow distribution rate.

STATEMENT OF GOVERNMENT INTEREST IN THE INVENTION

This application was made with government support under Department of Energy Grant No. DE-EE0002B95. The United States government may have certain rights in the invention.

TECHNICAL FIELD

This application is directed, in general, to cooling apparatus and methods and more particularly to controllable cooling apparatus and methods.

BACKGROUND

Electronically operated devices generate heat. Removal of the heat generated from such devices is often required to ensure satisfactory operation of the devices. One example of a cooling technique is pumped refrigerant-based liquid-cooling systems for cooling high-heat-density data centers and telecommunications equipment.

SUMMARY

Some embodiments of the disclosure provide a method for cooling a heat source using a refrigerant flow in a heat exchanger, the method including: (1) measuring a value corresponding to a property of the refrigerant flow indicative of an amount of heat being removed by the heat exchanger, (2) determining a flow distribution rate for the flow of the refrigerant in the heat exchanger using the measured value and (3) operating a valve to adjust the flow rate of the refrigerant in the heat exchanger according to the determined flow distribution rate.

Other embodiments of the disclosure provide an apparatus including a heat exchanger configured for cooling a heat source using a refrigerant, the apparatus including: (1) a flow property meter for measuring a value corresponding to a property of the refrigerant flow, (2) a processing unit coupled to the meter and configured to determine a flow distribution rate for the flow of the refrigerant in the heat exchanger using the measured value and (3) a valve coupled to the processing unit and configured to adjust the flow rate of the refrigerant in the heat exchanger according to the determined flow distribution rate.

BRIEF DESCRIPTION

Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is an exemplary schematic configuration of a modular cooling system including a heat exchanger according to known solutions;

FIG. 2 is an exemplary schematic representation of the implementation of the pumped-refrigerant system of FIG. 1 for use in a data center cooling system;

FIG. 3A shows the effect of heat load on the refrigerant flow rate through a heat exchanger and the vapor quality of refrigerant exiting the heat exchanger;

FIG. 3B shows the effect of variation in heat load on vapor quality and the boiling heat transfer coefficient along the length of heat exchanger;

FIG. 4 is an exemplary schematic configuration of a modular cooling system according to embodiments of the disclosure;

FIG. 5 is an exemplary schematic illustration of an assembly including an air-to-refrigerant heat exchanger assembled with a valve and a flow property meter;

FIGS. 6A and 6B illustrate an example of a capacitance operated flow property meter according to some embodiments;

FIG. 7 is an exemplary schematic representation of an embodiment of a measurement technique according to some embodiments; and

FIG. 8 is an exemplary schematic representation of an embodiment of a measurement technique according to some embodiments.

DETAILED DESCRIPTION

Pumped refrigerant-based liquid-cooling systems for cooling high-heat-density data centers and telecommunications equipment have a number of advantages over traditional air-based cooling approaches. Such typical advantages may include low energy use, high heat-carrying capacity and high reliability.

One cooling technique currently employed based on this technology involves modular cooling. In modular cooling, liquid refrigerant is pumped to multiple heat exchangers (HXs) typically in a parallel arrangement for the refrigerant plumbing. A heat exchanger typically removes the heat of a given shelf in an equipment rack. The amount of refrigerant flowing to the heat exchanger is typically determined by the pump and the flow resistance of the refrigerant distribution and return network, including the heat exchangers. However, in practice, the amount of heat dissipated by one device to be cooled (i.e., its heat load) may not be the same as that of another heat load in the same cooling network. Also a single heat load may have variations in the amount of heat dissipated thereby at different times during operation. Therefore, such passive determination of flow rate of the refrigerant as used in the known solution typically does not use any control in flow in response to variations and non-uniformities in one or more heat loads.

In the context of the present disclosure, the heat load of a device is understood to be the power (measured in Watts) that is dissipated by that device and which needs to be removed by the cooling system.

Some embodiments of the disclosure provide a method for cooling a heat source using a refrigerant flow in a heat exchanger, the method including: (1) measuring a value corresponding to a property of the refrigerant flow indicative of an amount of heat being removed by the heat exchanger, (2) determining a flow distribution rate for the flow of the refrigerant in the heat exchanger using the measured value; and (3) operating a valve to adjust the flow rate of the refrigerant in the heat exchanger according to the determined flow distribution rate.

According to some specific embodiments, the operation of the valve is controlled by a control unit or an operator. According to some specific embodiments, the value corresponding to a property of the refrigerant flow is determined by measuring a parameter selected from a group consisting of pressure, temperature, capacitance, thermal conductance, scattering of light, ultrasonic response, infrared or visible image, coolant vapor volume fraction, mass flow, power consumption, or a combination of such parameters. According to some specific embodiments, the value corresponding to a property of the refrigerant flow indicative of an amount of heat is determined by measuring a temperature difference across a reference element located between the heat source and the heat exchanger. According to some specific embodiments, a flow property value is determined by measuring a first differential pressure at a first location upstream of the heat exchanger and a second differential pressure measured at a second location downstream of the heat exchanger and determining a ratio between the differential pressure at the second location and the differential pressure at the first location. According to some specific embodiments, the method includes maintaining a refrigerant flow rate in a branch higher than an optimum flow rate wherein the optimum flow rate is a flow rate at which a heat load present in the branch is the exact heat load required to vaporize the entire refrigerant without superheat.

Some embodiments of the disclosure provide an apparatus including a heat exchanger configured for cooling a heat source using a refrigerant, the apparatus including a flow property meter for measuring a value corresponding to a property of the refrigerant flow, a processing unit for determining a flow distribution rate for the flow of the refrigerant in the heat exchanger using the measured value, and a valve for adjusting the flow rate of the refrigerant in the heat exchanger according to the determined flow distribution rate.

According to some specific embodiments, the apparatus further includes a control unit for controlling the operation of the valve. According to some specific embodiments, the apparatus includes a plurality of heat exchangers associated with respective heat sources included in a cooling cycle network. According to some specific embodiments, the apparatus includes a pump for pumping refrigerant through the heat exchanger. According to some specific embodiments, the cooling cycle network includes a plurality of branches wherein a branch includes at least one heat exchanger, a flow property meter and a valve. According to some specific embodiments, the apparatus further includes a valve for adjusting a flow of the refrigerant in more than one individual branch in the network. According to some specific embodiments, the flow property meter is configured for measuring a capacitance of a volume of the refrigerant in a refrigerant conduit. According to some specific embodiments, the flow property meter is configured for measuring a pressure within a volume of the refrigerant in a refrigerant conduit. According to some specific embodiments, the flow property meter is configured for measuring a thermal conductance within a volume of the refrigerant in a refrigerant conduit. According to some specific embodiments, the flow property meter is configured for measuring the scattering of light within a volume of the refrigerant in a refrigerant conduit. According to some specific embodiments, the flow property meter is configured for measuring an ultrasonic response within a volume of the refrigerant in a refrigerant conduit. According to some specific embodiments, the flow property meter is configured for measuring an infrared or visible image response within a volume of the refrigerant in a refrigerant conduit. According to some specific embodiments, the flow property meter is configured for measuring a volume of the refrigerant in a refrigerant conduit. According to some specific embodiments, the apparatus includes a temperature meter for measuring a temperature in a heat exchanger or in a heat source. According to some specific embodiments, the flow property meter includes a first differential pressure meter for measuring a differential pressure at a location upstream the heat exchanger and a second differential pressure meter for measuring a differential pressure at a location downstream of the heat exchanger. According to some specific embodiments, the apparatus is configured for maintaining a refrigerant flow rate in a branch higher than an optimum flow rate wherein the optimum flow rate is a flow rate at which a heat load present in the branch is the exact heat load required to vaporize the entire refrigerant without superheat.

It is to be noted that throughout the present disclosure including the claims the term, “valve,” is to be construed broadly such that the term may not only include conventional valves as commonly known, but also any other mass-flow controller device which is capable of adjusting the flow rate of a refrigerant in a heat exchanger.

FIG. 1 illustrates an exemplary schematic configuration of elements of a modular cooling system 1 using the technique described above. In this configuration, an air-to-refrigerant microchannel heat exchanger (HX) 10, typically including fins for heat dissipation, is placed at the air exhaust regions in individual shelves 11 of equipment racks of a typical electronically operated system (a rack being a cabinet that typically contains several shelves). It is to be noted that the system of FIG. 1 is only one example of a pumped refrigerant approach provided for illustrative purposes, and that other configurations may also exist, such as refrigerant-cooled cold plates that provide direct component-level cooling in circuit packs within equipment racks.

Referring back to FIG. 1, liquid refrigerant, such as one known as R134a (1,1,1,2-Tetrafluoroethane, a well-known and widely-used refrigerant), is pumped to the finned microchannel heat exchanger 10 which is placed at the air exhaust of the shelf 11. In the example of FIG. 1 the heat exchanger 10 has an inlet conduit 12 for the flow of the refrigerant into the heat exchanger structure 10 and an outlet conduit 13 for the flow of the refrigerant out of the heat exchanger structure 10.

A fan tray (not shown) may be used to push cold ambient air through the shelf 11, where air gains heat dissipated by the electronic components in the shelf 11. This flow of air is shown in FIG. 1 by way of arrow A1. The heated air the passes through the shelf 11 and the heat exchanger 10, as shown by way of arrows A2 and A3, and exits the heat exchanger, typically at a cooler temperature, due to the heat exchange between heated air and refrigerant. The heated, and thus partly or completely vaporized refrigerant is typically returned to a refrigerant pump system (not shown) for condensation via heat exchange with, for example, building chilled water or a secondary loop having a compressor, and the heat is rejected to the external ambient environment.

It is to be noted that fans may also operate to pull air from the shelf through a fan tray, as well as operate in “push/pull” mode in which two or more fan trays, on different sides of the heat source cooperate to move the air in a determined direction.

FIG. 2 shows an exemplary schematic representation of the implementation of the modular pumped-refrigerant system of FIG. 1 for use, for example, in a data center cooling system 2. The cooling system 2 includes a pumping mechanism 21 and a cooling cycle network 22. The pumping mechanism 21 may include a refrigerant pump 212 and a cool (chilled) liquid supply pump 214. The refrigerant pump 212 and the cool liquid supply pump 214 may be included in a cool refrigerant heat exchanger 216. The cooling liquid for use by pump 214 may be, for example, water. The cooling cycle network 22 includes an array of heat assemblies 222-1, . . . , 222-N (generally 222-i). Herein, a heat assembly is to be understood to include a shelf containing electronic devices and a respective heat exchanger. The heat exchangers operate to transfer heat away from (or to cool) the shelf using a refrigerant as described with reference to FIG. 1.

The heat assemblies 222-i may be installed in parallel configuration in the network 22 as shown in FIG. 2; however this is only exemplary, and other configurations may also be used.

In operation, the heat assemblies 222-i may be supplied with refrigerant, for example R134a, by the refrigerant pump 212 through an input branch 224-1. The refrigerant is then branched from the input branch 224-1 to individual heat assemblies 222-i through individual branches 226-1, . . . , 226-N (generally 226-i). The refrigerant is subsequently output from the individual heat assemblies 222-i and the individual branches 226-i and is returned to the pump 21 through output branch 224-2. Heat exchangers may be placed at the air exhausts of respective shelves in equipment racks (11 in FIG. 1). As mentioned before, in a typical application of this system, heat loads may vary from shelf to shelf (or from branch to branch). In practice, the flow rate of the refrigerant flowing through the heat exchangers with high heat load may be lower than the flow rate of the refrigerant flowing through the heat exchangers with low heat load. Typically, superheating of a refrigerant occurs faster in the heat exchangers with high heat load than in heat exchangers with low heat load, resulting in lower heat transfer performance. These effects can cause a positive feedback loop (increasing temperature after each cycle) that starves the high-heat-load shelves of refrigerant (i.e., insufficient refrigerant to cool the heat load).

The term “superheat” as used in the context of the present disclosure refers to a status in which the temperature of the heated fluid increases above its saturation (evaporation) temperature and the liquid is totally evaporated. The superheat is a parameter to gauge system's performance. The solution proposed herein is aimed at preventing the working fluid from complete vaporization.

FIG. 3A shows the effect of heat load on the refrigerant flow rate through a heat exchanger and the quality of refrigerant exiting the heat exchanger at constant pressure differential. In FIG. 3A, the X-axis (abscissa) represents heat load in the shelf and the Y-axis (ordinate) represents refrigerant flow rate. No units are used in the figure because the representation is only provided for illustrative purposes.

As shown in FIG. 3A, as the heat load in a shelf increases, an increasing fraction of the refrigerant is converted to vapor. In the partially- or fully-vaporized state, the refrigerant has a higher kinematic viscosity than in the purely liquid state. As represented by the dotted curve, at higher heat load values the vapor quality of the refrigerant reaches a constant value which represents the fact that, at such sufficiently high heat loads, substantially all the liquid refrigerant is vaporized in its path through the heat exchanger. At that point, the ability of the refrigerant to absorb heat is significantly reduced because the heat capacity of the vapor is typically much smaller than the latent heat capacity of the phase transformation.

Herein the refrigerant vapor quality is to be understood to refer to the mass fraction of the refrigerant that has been converted from liquid to vapor. Likewise, the void fraction as referred to herein is considered to refer to the fraction of the volume that is occupied by vapor bubbles.

At the same time, the partially- or totally-vaporized refrigerant has a higher kinematic viscosity than liquid refrigerant. This leads to higher flow impedance and thus to lower flow rate, degrading the thermal performance in a positive feedback loop. The decrease of flow rate associated to the increase of the heat load is shown in FIG. 3A using a solid curve.

Referring now to FIG. 3B, the effect of variation in heat load on vapor quality and the boiling heat transfer coefficient along the length of a heat exchanger is shown. In FIG. 3B the X-axis (abscissa) represents distance in the flow path of a refrigerant along a heat exchanger and the Y-axis (ordinate) represents heat transfer coefficient. Curves 31, 32 and 33 are illustrated with respective heat load amounts being such that the heat load of curve 31 is higher than the heat load of curve 32 and the heat load of the latter is higher than the heat load of curve 33. No units are used in the figure because the representation is only provided for illustrative purposes.

It may be observed that as the heat load in the heat assembly increases, the vapor quality in the heat exchanger reaches level 1 (complete vaporization) sooner (i.e., further upstream) than at lower heat load. In addition to having higher flow impedance, the full-vapor condition downstream in the HX typically has lower heat transfer performance than the two-phase (vapor-liquid) flow in the upstream part of the HX because the heat capacity of the vapor is much less than that of the phase change. It is to be noted however that, for the portion of the curve representing the boiling heat transfer coefficient corresponding to vapor quality less than 1, the shape of the curve shown in FIG. 3B is idealized, whereas in actuality there may be significant variation in the boiling heat transfer coefficient depending on the particular boiling regime (bubbly, annular, etc.) associated with the heat transfer. The idealization of the curve is merely intended to illustrate the substantial differences in heat transfer between the boiling regime and the superheated regime.

In addition to these reductions in heat-transfer efficiency, system operation at high vapor quality may in some occasions damage the refrigerant pump and reduce overall reliability and heat transfer performance.

In one known solution the refrigerant flow is controlled by placing bypass valves that allow a fraction of the refrigerant leaving the pump to bypass the refrigerant-distribution network, when needed, and instead to flow immediately into the refrigerant return network that resupplies the pump. This approach is typically attempted with the aim of compensating limitations of certain refrigerant pumps in accommodating a wide range of heat loads. However, this known solution does not address performance issues related to non-uniform heat loads in the network of heat exchangers which are supplied with refrigerant.

Another known solution uses thermal expansion valves (TEVs) which are a technology commonly used in refrigeration systems to control the amount of refrigerant superheat leaving a heat exchanger. This is to ensure that no liquid exits the HX, which can damage the compressor which receives it, and to maintain a proper amount of superheat for energy-efficient operation of the system.

Thermal expansion valves however are typically not adequate for pumped refrigerant applications because it is often desirable that the refrigerant exits the HX in a saturated state, e.g., with a portion of refrigerant still in the liquid phase.

As already discussed above, the heat transfer performance of a HX is determined by the refrigerant flow rate and the vapor quality in the HX, which is affected by the existence of different heat loads in the racks within a cooling network. Therefore, a dynamic control and feedback mechanism is desired for enhancing the overall thermal performance and reliability when using HX for cooling multiple equipment racks.

Some embodiments of the disclosure aim at providing a mechanism that is capable of dynamically redistributing the refrigerant flow in different HXs within a cooling network taking into account the individual shelf heat loads and variations thereof.

Such a mechanism allows for increasing the flow of the refrigerant in heavily loaded HXs while decreasing the flow in lightly loaded ones, or for performing flow adjustments in the various branches of the cooling network according to specific needs.

Such a redistribution of the refrigerant flow would be particularly beneficial in systems using a refrigerant working in the two-phase regime. In such systems, the removed heat causes a certain amount of refrigerant to vaporize, and the vapor bubbles thus produced increase the flow impedance. In the standard parallel configuration of cooling networks, as already discussed, the amount of refrigerant flowing in a given HX decreases as the heat load increases which leads to lower heat transfer capacity in precisely those circumstances where it needs to be higher.

According to some embodiments, a feedback control configuration is provided in the pumped-refrigerant cooling system.

In some specific embodiments, a flow property meter is provided in a feedback control configuration to detect the refrigerant vapor quality exiting each HX. Information on the vapor quality is used in the feedback control operation, e.g., using an algorithm to adjust the quantity of refrigerant to be pumped into each HX.

In other embodiments, other diagnostic signals, such as the pressure drop across each HX, the temperature increase across the HX, or the refrigerant mass flow rate through the HX, may be used in the feedback control to detect vapor quality. The use of such information in a properly designed feedback control configuration enables an optimal distribution of the refrigerant flow among heat exchangers so as to adapt to different heat load conditions.

It is to be noted that, although some embodiments are provided herein in the context of the exemplary pumped refrigerant system, the invention is not to be construed as limited to only such applications and may be applicable to any system having a refrigerant distribution network.

Referring now to FIG. 4, an embodiment of the disclosure is shown in which a feedback control configuration is provided in a pumped-refrigerant cooling system 4.

The cooling system 4 of FIG. 4 includes a pumping mechanism 41 and a cooling cycle network 42. The pumping mechanism 41 may include a refrigerant pump 412 and a cool (chilled) liquid supply pump 414. The refrigerant pump 412 and the cool liquid supply pump 414 may be included in a cool refrigerant heat exchanger 416. The cooling liquid for use by pump 414 may be, for example, water. The cooling cycle network includes an array of heat assemblies 422-1, . . . , 422-N (generally 422-i). Similar to FIG. 2, a heat assembly is to be understood to include a shelf containing electronic devices (i.e., heat source) and a respective heat exchanger.

The heat assembly 422-i may be installed in parallel configuration in the network 42 as shown in FIG. 4; however, this is only exemplary and other configurations may be used.

In operation, the heat assemblies 422-i may be supplied with refrigerant, for example R134a, by the refrigerant pump 412 through an input branch 424-1. The refrigerant is then branched from the input branch 424-1 to individual heat assemblies 422-i through individual branches 426-1, . . . , 426-N (generally 426-i). The refrigerant is subsequently output from the heat assemblies 422-i and the individual branches 426-i and is returned to the pump 41 through output branch 424-2. Heat exchangers may be placed at the air exhausts of respective shelves in equipment racks (11 in FIG. 1).

As already mentioned above, in a typical application of this system, due to variation in heat loads from shelf to shelf (or from branch to branch), the heat exchangers may provide different levels of heat transfer.

According to the present disclosure, individual valves 428-1, . . . , 428-N (generally 428-i) for adjusting the refrigerant flow into the individual heat exchangers may be installed on the inlet side of the heat exchangers 422-i. Furthermore, use is made of flow property meters so as to infer the refrigerant quality output from the heat exchangers 422-i.

As used herein, the term, “flow property,” is to be understood to refer to at least one property of the refrigerant as it flows through the heat exchanger. By way of non-limiting examples, the property of the refrigerant to be measured as it flows through a heat exchanger may be selected from any one of the following properties or a combination thereof: pressure, temperature, capacitance, thermal conductance, scattering of light, ultrasonic response, infrared or visible image, mass flow rate, power consumption. These properties are equivalent in the sense that they can be measured and provide at least some information regarding the amount of heat in the heat exchanger. Therefore a value obtained as a result of the measurement of the flow property may be associated to, and thus indicative of, an amount of heat in the heat exchanger.

Referring back to FIG. 4, flow property meters 430-1, . . . , 430-N (generally 430-i) may be installed on the refrigerant outflow. In one example a flow property meter may be configured to measure electrical capacitance across a fluid within a piping conduit, and use this electrical capacitance value to infer a void fraction or vapor quality.

Optionally, additional valves may be installed in the cooling cycle network 42. For example a valve 440 may be installed on the main line to control flow in the entire network in case of need; or bypass valve 450 may be installed at a position where in case of need the flow of the refrigerant may be bypassed from the heat exchangers. In the parallel network configuration of FIG. 4 such a bypass valve 450 is installed in parallel configuration with respect to the branches 426-i. The main valve and the individual valves on each branch may be adjusted based on the property measurement of the flow and the vapor quality information inferred from such measurement, to ensure sufficient refrigerant flow into the heat exchangers with high heat load and thus prevent refrigerant dryout and superheat in the heat exchangers 422-i. Herein, the term dryout refers to total evaporation of the liquid.

In one embodiment, a processing unit 460 may be used to collect data related to the measurements of the individual flow property meters 430-i and any other feedback signals and is configured to compute the optimum flow distribution among the heat exchangers. A control unit 470 may then modify the flow in the valves accordingly. The processing unit may be any device available and suitable for the intended use such as a personal computer, server or other devices with functions to store, compute and/or process, a central processing unit or an FPGA. Likewise a control unit may be any device available and suitable for the intended use such as a computer-based data acquisition and feedback system.

In another embodiment, individual feedback control configurations, i.e., combination of a processing unit and a control unit, may be implemented on all or some branches 426-i. In such cases, each valve on such branches is adjusted according to a corresponding individual measurement performed by the individual processing unit and thereby controls the flow in a corresponding heat assembly 422-i as desired.

FIG. 5 is an exemplary schematic illustration of an assembly 5 including an air-to-refrigerant heat exchanger 51 assembled with a valve 52 and a flow property meter 53. The heat exchanger 51 may have an input path 54 connected to the valve 52 and an output path 56 connected to the flow property meter 53. Further, the heat exchanger 51 may include fluid channels 58 including fins 59 for dissipating heat.

The valve 52 may be any known continuously-adjustable control valve such as a metering valve or a needle valve or a refrigerant-grade control valve with stepper-motor actuation.

The flow property meter 53 may be one configured to measure the flow property present in the refrigerant output path by measuring parameters in the refrigerant flow such as pressure, temperature, thermal conductivity, capacitance, scattering of light, ultrasonic response, infrared or visible image, and the like.

An example of a capacitance or voltage operated flow property meter is shown in FIGS. 6A and 6B. In this particular example, the flow property meter is configured to measure the impedance on the flow of the refrigerant. The measurement of the flow property in the assembly may also be made by using a combination of such measurement parameters.

In one example, the flow property meter 52 may include electrodes or pins that are inserted inside the refrigerant input path 54. In another example, the flow property meter 52 may include metal plates or foils on the surface of the path 54 in which case it does not need to enter in contact with the liquid inside the path 54. As the dielectric constant of liquid refrigerant is higher than that of the vapor, the capacitance of a fixed volume of refrigerant depends strongly on the vapor quality.

With this configuration, as already discussed in relation to FIG. 4, the measurements corresponding to each branch may be provided to the processing unit which computes the optimum or desired flow distribution in the network and provides such data to the control unit which in turn performs the required operations on the valves to control the flow rate in the branches. Instead of using a control unit, the control may also be made manually by an operator.

FIGS. 6A and 6B respectively illustrate exemplary representations of a side view and a front view of a flow property meter 6. In this example the flow property meter 6 is of capacitance type. Electrodes 61-a and 61-b may be inserted into the tube (or pipe) 62 through which refrigerant may flow. Electrodes 61-a and 61-b may then be connected to an electrical circuit (not shown) for performing measurements and data collection. The physical principle underlying operation of this flow property meter is based on the dependence of the capacitance of the enclosed volume on the vapor fraction. The electrodes may be connected in any suitable configuration. For example in FIGS. 6A and 6B, positive and negative electrodes are installed in a diagonal configuration, however other configurations such as one having neighboring positive and negative electrodes may also be used.

It is to be noted that the use of electrodes inserted in the tube 62 of the flow property meter for performing measurements in the particular embodiment illustrated in FIGS. 6A and 6B is only exemplary and other configurations for measuring flow properties may be employed; for example electrodes may be placed on the outer surface of the tube 62 without being inserted therein.

Signals for feedback control may also be generated by measuring the mass flow at each branch (e.g., at the inlet of the heat-exchanger), or the temperatures at particular points in the HX or the shelf (e.g., the hottest component in the shelf), or the power consumption of the shelves, or the heat-load on each heat-exchanger, or an appropriate combination of pressure measurements on each line (e.g., the ratio between the differential pressures at the outlet and inlet of the HX).

FIG. 7 is an exemplary schematic representation of another possible measurement technique to determine a signal for the feedback control. Heat exchangers 72-1 and 72-2 are shown to be installed in parallel configuration with respect to each other and are configured to receive refrigerant entering from an input branch 74-1 and branched into individual branches 76-1 and 76-2 connected to heat exchangers 72-1 and 72-2 respectively. The heat exchangers 72-1 and 72-2 are further connected at their respective output to the individual branches 76-1 and 76-2 which in turn merge into an output branch 74-2 from which the refrigerant output from the heat exchangers may flow back to a pump (not shown) for further recycling of the refrigerant. According to this embodiment, heat load associated to a heat exchanger, in this case heat exchanger 72-1, may be determined by a temperature differential measurement across a reference element 73. The reference element 73 may be located at any convenient location between the heat source and the heat exchanger 72-1. The temperature differential may be measured by thermocouples, Platinum resistance thermometers, thermistors or other temperature sensors, represented in FIG. 7 by reference numeral 75. Based on the value of the measured temperature differential, one may be able to determine the heat load of the heat assembly. On the other hand, the operating conditions of the heat exchanger, such as, for example, its current refrigerant flow rate and its heat removal capacity, may be known for a particular implementation. Therefore, by determining the heat load in the assembly and correlating this data with information corresponding to the operating conditions of the heat exchanger, one may obtain an estimation of the amount of the heat that needs to be removed from the heat exchanger operating under that particular flow rate. This estimation may therefore be considered indicative of the flow property value of the refrigerant flowing in the heat exchanger.

In other embodiments, instead of estimating the power dissipation as mentioned above, such dissipation may be directly computed from the voltage drop across and the current through the device being cooled. In other embodiments, instead of measuring the power dissipated by a given device, its temperature may be measured directly.

In this manner, if the value of the differential measurement shows an increase from a reference value, it may be determined that the heat load is increasing—thereby affecting the flow. Therefore, by performing control actions—for example, by opening the respective individual valve—one may allow higher flow of refrigerant in the branch and thereby increasing the cooling rate.

The above configuration is one example of determining the heat load. If this process is performed locally, some degree of control becomes available related to a heat exchanger in a branch. It is also possible to include a flow property meter upstream the heat exchanger such that, by knowing the refrigerant flow and the amount of the heat load, one may be able to calculate the vapor quality at the outlet of the heat exchanger.

In case of a global control scheme, e.g., using a central unit to process all the data obtained in a network cooling cycle network such as that of FIG. 4, in principle one may calculate the refrigerant flow property on each branch measuring the heat loads associated with a corresponding heat exchanger, provided the change in flow resistance as a function of the heat load and flow is known.

FIG. 8 is an exemplary schematic representation of another possible measurement technique to determine a signal for the feedback control. Heat exchangers 82-1 and 82-2 are shown to be installed in parallel configuration with respect to each other and are configured to receive refrigerant entering from an input branch 84-1 and branched into individual branches 86-1 and 86-2 connected to heat exchangers 82-1 and 82-2 respectively. The heat exchangers 82-1 and 82-2 are further connected at their respective output to the individual branches 86-1 and 86-2 which in turn merge into an output branch 84-2 from which the refrigerant output from the heat exchangers may flow back to a pump (not shown) for further recycling of the refrigerant. According to this embodiment, a first differential pressure is measured at a first location 87 upstream the heat exchanger 82-1 and a second differential pressure is measured at a second location 88 downstream the heat exchanger 82-1. The ratio between the differential pressure at the second location 88 over the differential pressure at the first location 87 (which may be used a reference), is a function of the vapor quality. This ratio is independent of the unknown mass flow of the refrigerant and may be used as indicative of the flow property of the refrigerant as it flows through the heat exchanger 82-1. For example, if the value of the ratio shows an increase from a reference value, it may be determined that vapor quality is increasing and thereby perform control actions, for example, by opening the respective individual valve to allow higher flow of refrigerant in the branch.

In another embodiment of the disclosure, the feedback control configuration (including the pump mechanism and the cooling cycle network) may be used to redistribute the refrigerant among different branches in a rack or different components on a branch. A similar technique as described with reference to FIG. 4, 5, 6 or 7 may be used to achieve such behavior.

In addition to the various embodiments for dynamically adjusting the flow of the refrigerant as described above, a further safety measure may be employed in the cooling process which may include the following principles. The temperature difference between the input and output refrigerant for a heat exchanger is measured. If the measured output temperature is higher than the measured input temperature, it may be concluded that the refrigerant has vaporized completely, and the vapor is getting hot. In this condition, the control unit may be configured to increase the refrigerant flow rate to that heat exchanger until the temperature increase is reduced below a pre-determined threshold value.

In this manner, a pumped-refrigerant-based system may be provided that is capable of dynamically adjusting the flow of the refrigerant through one or more heat exchangers so as to provide only the necessary or desired amount of refrigerant for dissipating a given heat load associated with the respective heat exchanger. This allows non-uniform distributions of refrigerant among non-uniform heat loads within the system, and thereby overcomes a limitation of conventional implementations of pumped-refrigerant technology. The proposed solution also allows achieving either locally or globally optimal distributions of refrigerant as desired in each specific application so as to maximize or at least improve the heat removing capacity of the system.

Furthermore, the proposed solution allows one to incorporate margin into the design, which is effective for handling any potential power excursions. Maintaining a margin is useful for the following reasons. In principle, one may be able to adjust the refrigerant flow rate to each branch such that its present heat load is exactly what is required to vaporize all the refrigerant (i.e., achieve vapor quality 1.0) without superheat (i.e., with output temperature becoming equal to input temperature). However, operation under such optimum or extreme condition may not be preferred because any sudden increase in heat load will cause more superheat (i.e., raises the temperature of the output refrigerant vapor). By the time the system detects the superheat increase, responds so as to increase the heat removal and the excess power is finally cooled, the devices being cooled may have heated too much. Therefore, it may be preferable to maintain a degree of margin in the operation by running the system at a refrigerant flow rate slightly higher than the above optimum flow rate. By doing so, a sudden increase in heat load may only vaporize more refrigerant without raising its temperature to a degree that the refrigerant has vaporized completely. As the cooling process takes time the proposed margin allows the system to respond and adjust the flow rate before temperatures has increased too much.

Those skilled in the art to which this application relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments. 

What is claimed is:
 1. A method of cooling a heat source using a refrigerant flow in a heat exchanger, comprising: measuring a value corresponding to a property of said refrigerant flow indicative of an amount of heat being removed by said heat exchanger; determining a flow rate of the refrigerant in the heat exchanger using said value; and operating a valve to adjust said flow rate of said refrigerant in said heat exchanger according to said determined value.
 2. The method as recited in claim 1, wherein said measuring comprises measuring at least one parameter relative to said heat exchanger and selected from a group consisting of: pressure, temperature, capacitance, thermal conductance, scattering of light, ultrasonic response, infrared image, visible image, mass flow, and power consumption.
 3. The method as recited in claim 1 wherein said measuring comprises measuring a temperature difference across a reference element located between said heat source and said heat exchanger.
 4. The method as recited in claim 1 wherein a flow property value is determined by measuring a first differential pressure at a first location upstream of said heat exchanger and a second differential pressure measured at a second location downstream of said heat exchanger and determining a ratio between said second differential pressure and said first differential pressure.
 5. The method as recited in claim 1 further comprising maintaining a refrigerant flow rate in a branch of said heat exchanger higher than an optimum flow rate, said optimum flow rate being a flow rate at which a heat load present in said branch is a heat load required to vaporize an entirety of said refrigerant without superheat.
 6. An apparatus comprising a heat exchanger configured to cool a heat source using a refrigerant, comprising: a flow property meter configured to measure a value corresponding to a property of a flow of said refrigerant; a processing unit coupled to said flow property meter and configured to determine a flow distribution rate for said flow of said refrigerant in said heat exchanger using said value; and a valve coupled to said processing unit and configured to adjust a flow rate of said refrigerant in said heat exchanger according to said determined flow distribution rate.
 7. The apparatus as recited in claim 6, further comprising a control unit coupled to and configured to control an operation of said valve.
 8. The apparatus as recited in claim 6, further comprising a plurality of heat exchangers associated with respective heat sources comprised in a cooling cycle network.
 9. The apparatus as recited in claim 6, further comprising a pump configured to pump refrigerant through said heat exchanger.
 10. The apparatus as recited in claim 8 wherein the cooling cycle network comprises a plurality of branches, wherein a branch includes: at least one heat exchanger; a flow property meter; and a valve.
 11. The apparatus as recited in claim 11, further comprising a valve configured to adjust a flow of said refrigerant in more than one individual branch in said network.
 12. The apparatus as recited in claim 10 wherein said flow property meter is configured to measure a capacitance of a volume of said refrigerant in a refrigerant conduit.
 13. The apparatus as recited in claim 10 wherein said flow property meter is configured to measure a pressure within a volume of said refrigerant in a refrigerant conduit.
 14. The apparatus as recited in claim 10 wherein said flow property meter is configured to measure a thermal conductance within a volume of said refrigerant in a refrigerant conduit.
 15. The apparatus as recited in claim 10 wherein said flow property meter is configured to measure a scattering of light within a volume of said refrigerant in a refrigerant conduit.
 16. The apparatus as recited in claim 10 wherein said flow property meter is configured measure an ultrasonic response within a volume of said refrigerant in a refrigerant conduit.
 17. The apparatus as recited in claim 10 wherein the flow property meter is configured to measure an infrared or visible image response within a volume of said refrigerant in a refrigerant conduit.
 18. The apparatus as recited in claim 10 wherein said flow property meter is configured to measure a vapor or void fraction of said the refrigerant within a volume of said refrigerant in a refrigerant conduit.
 19. The apparatus as recited in claim 6 wherein said flow property meter comprises a first differential pressure meter configured to measure a differential pressure at a location upstream a flow of said refrigerant relative to said heat exchanger and a second differential pressure meter configured to measure a differential pressure at a location downstream said flow of said refrigerant relative to said heat exchanger.
 20. The apparatus as recited in claim 6 wherein said apparatus is configured to maintain a refrigerant flow rate in a branch of said heat exchanger higher than an optimum flow rate, said optimum flow rate being a flow rate at which a heat load present in said branch is a heat load required to vaporize an entirety of said refrigerant without superheat. 